An urban legend holds that, if a student arrives on time for class, and the teacher does not arrive within ten minutes of the official class start time, then the student is entitled to skip that class for that day without receiving an unexcused absence. Few, if any, schools actually have such a rule; yet the myth endures because of the student appeal of such a rule.
The rule is not necessarily alleged to be ten minutes. The "five minute rule" or the "fifteen minute rule" have also been used in this same sense.
What the fuck, it's 9:08 and Kotter's not here yet! Two more minutes and I'm invoking the ten minute rule.